1. Field of the Invention
The present invention relates to an electrostatic image developing toner for developing a latent electrostatic image formed in an electrophotographic method, an electrostatic recording method and an electrostatic printing method, a toner container containing the toner, a developer, an image forming apparatus, an image forming method and a process cartridge.
2. Description of the Related Art
Dry-process developing units using a powdery developing agent have widely been employed in image forming apparatuses such as electronic copiers, printers and facsimiles, in which a latent electrostatic image formed on a latent image bearing member is visualized with a developer to obtain a recorded image.
In recent years, color image forming apparatuses using electrophotographic process have broadly been employed, and digitized images are easily available. Thus, it is required to make an image to be printed at higher definition. While studying higher resolution and gradation of an image, as an improvement of a toner which visualizes a latent image, it has been studied to further conglobate and minimize in particle size for forming the image at high definition. And, since in the toners produced by the pulverizing methods, their conglobation and minimization are limited, so-called polymerized toners produced by a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method capable of conglobating and minimizing in particle size have been being employed.
Polymerization toners have a small particle diameter and thus, exhibit an increased adhesion force to members, which degrades transfer efficiency and causes filming. Also, the polymerization toners have a spherical shape and thus, are poor in cleanability. In addition, the polymerization methods allow toner materials of relatively low resistance to be localized near the toner surfaces. Therefore, the formed polymerization toners involve background smear due to their low chargeability. Meanwhile, in recent years, there has been increased demand for toners that attain high-quality images and have low-temperature fixing property for energy saving. Thus, a binder resin having a low melt temperature is desirably used. However, toners having a low-temperature fixing property possess newly arising problems such as generation of blocking at high-temperature, high-humidity environment, which is associated with degradation in heat resistance storage stability.
In view of this, attempts have been made to modify the surfaces of toner core particles to solve the aforementioned problems. The method for surface modification is, for example, dry methods in which fine particles are made to adhere onto the toner surfaces by the action of mechanical impact, and wet methods in which a resin dispersing agent is added to a dispersion liquid containing toner particles dispersed in a solvent, wherein the resin of the resin dispersing agent is different from the resin forming the toner particles. Regarding the dry methods, Japanese Patent (JP-B) No. 2838410 or other literatures disclose a toner including base particles and fine particles embedded in the surfaces thereof, wherein the toner is produced by adding the fine particles to the base particles heated to a temperature near their softening point, followed by stirring and mixing. Also, JP-B No. 2750853 discloses a toner including fine resin particles and toner core particles which are covered with the fine resin particles by the action of mechanical impact. In these dry methods, the fine particles are ununiform and thus cannot be attached on the toner surfaces sufficiently. As a result, the fine particles are exfoliated to cause problems such as filming and adhesion.
Regarding the wet methods, Japanese Patent Application Laid-Open (JP-A) No. 2008-090256 or other literatures disclose a method in which the surfaces of toner core particles formed of first resin particles and a colorant are partially or totally covered with second resin particles. However, according to this method, the toner core particles are covered with the second resin particles so sparsely and ununiformly that background smear and storage stability cannot be sufficiently improved, although cleanability is improved. In addition, degradation of transferability occurs.
JP-A No. 2008-233430 or other literatures disclose a toner including toner core particles and convex portions with an average diameter of 100 nm to 500 nm which are provided on the surfaces of the toner core particles, wherein the toner core particles are covered with the convex portions at a coverage rate of 10% to 80%. However, according to the production method described in Examples, the protrusions of the toner are not uniform in size, and thus the toner cannot solve problems such as background smear. The binder resin forming the convex portions has high polarity to greatly change depending on the environment and thus, is insufficient in improvement of heat resistance storage stability.
JP-A No. 2003-202701 or other literatures disclose a method in which fine resin particles are added in advance to an aqueous phase for fusion to control the particle diameter. However, in this method, the fine resin particles are incorporated into toner core particles, and as a result, the toner core particles cannot be covered with the fine resin particles in such an amount that heat resistance storage stability is improved.
According to JP-A No. 09-258480, cores are totally covered with shell layers, leading to considerable degradation of fixing property.
Presumably, toners or toner-containing cartridges are transported under application of a certain pressure. Thus, simply by increasing the glass transition temperature of the toner particle surface through surface modifications, the toner unavoidably deforms due to pressure at a high-temperature, high-humidity environment. Therefore, care should be taken on the glass transition temperature of the toner core particles. It cannot be stated that any of the above patent literatures can attain both desired low-temperature fixing property and desired heat resistance storage stability under application of a certain pressure. For example, JP-A Nos. 2001-175025 and 2007-003840 made attempts to improve heat resistance storage stability using fine resin particles. However, since the glass transition temperature of toner core particles is low, the toner deforms due to application of pressure, indicating that only fine resin particles existing in the outer layer cannot improve storage stability under application of pressure.